Various approaches to develop new and more specific chemical lignin depolymerizing processes for industrial purposes are reported in the literature. G. C. Hobbs and J. Abbot (J. Wood Chem. Technol., 14, 195-125, 1994) tried to use free copper ions as additive in peroxide bleaching without the application of a coordination compound to bleach Eucalyptus regnans SGW pulp. The addition of free copper ions did not improve the result: the brightness increase in a three-hour peroxide bleaching stage using copper was 17.6% compared to 18.4% without copper. No results are reported on the reduction of kappa or viscosity values.
In some cases the idea was to mimic the redox cycle of the lignin polymerizing/depolymerizing enzymes. Laccase is a copper containing enzyme which is oxidized by molecular oxygen while lignin peroxidase and manganese peroxidase are iron containing enzymes, oxidized by hydrogen peroxide. No copper containing enzyme, oxidized by hydrogen peroxide is known. Related to lignin degradation, copper is a transition metal constituting the catalytic site of laccase. Copper also constitutes the catalytic site of various other metal proteins such as galactose oxidase, tyrosinase, ascorbate oxidase, superoxide dismustase, nitrite reductase, hemocyanin and plastocyanin. In relation to this invention a system mimicking the catalytic function of laccase reported by S. Kawai and H. Ohashi (Holzforschung, 47, 97, 1993) has to be mentioned. Dimeric lignin model compounds were decomposed in the organic solvent dichloromethane by a copper-amine complex. This reaction does not include peroxide in the catalytic cycle and for that reason cannot be compared with the reaction described in this invention.
A reaction of copper with lignin including peroxide has been reported by M. M. Santos et al. (J. Braz. Chem. Soc., 6, 257, 1995). The so-called Gif system has been developed originally to achieve the oxygenation of hydrocarbons. The reaction medium of the Gif system is invariably a mixture of pyridine and carboxylic acid, in which carboxylic acid, as well as the organic solvent pyridine has to be present in large excess (80% solution). Santos et al. tried to use the Gif system to decompose isolated lignin from different pulps with Cu(II) plus Fe (II) in a pyridine/acetic acid solution under an inert atmosphere. Under these conditions, phenolic degradation products were analyzed, but concomitantly a high amount of toxic pyridine derivatives such as 2,2'-bipyridyl, 2,3'-bipyridyl, methyl pyridine and pyridinone are yielded. The high amount of pyridine per se as well as the creation of large amounts of toxic derivatives makes it impossible to utilize this process for industrial purposes, as stated by the authors.
Oxidation of lignin model compounds with copper and peroxide in a 83% acetic acid solution as solvent has been reported by Van-Ba Huynh (Biochem. Biophys. Res. Commun., 139, 1104, 1986). This reaction was carried out at refluxing temperature above 100.degree. C.
Only the last two processes can be compared to some extent with the present invention as peroxide is used and organic compounds are added to the lignin degrading copper/peroxide system. The difference between the two methods and the method described in this invention is that the former reactions are carried out in organic solvents above 80% concentration, while in this invention the depolymerization of lignin by copper/peroxide is claimed to take place in aqueous solutions with addition of only low amounts of various copper coordination compounds. These conditions can be regarded to be a prerequisite for an industrial application.
Another reaction, very different from the one claimed here, but mimicking biological processes is the reaction of Fe2+ with hydrogen peroxide, known as Fenton reaction, which is carried out by brown rot fungi and decomposes preferably the carbohydrates in lignocellulosic material. Likewise, manganese is known as a degrader of lignin when oxidized to Mn (III) by manganese peroxidase.